The weird part: There was no relevant news or obvious reason for the slump—Hillary Clinton hadn’t tweeted about regulating drug prices—leaving investors and analysts scratching their heads for an explanation. The S&P Pharmaceuticals Select Industry Index fell more than 3% Thursday, with only five stocks actually trading up, while the Nasdaq Biotechnology Index sank nearly 4%. The Dow Jones Industrial Average was up 125, on the other hand, or nearly 1%.

Individual drug companies suffered even more. Valeant Pharmaceuticals (VRX), whose stock was punished several months ago amid regulatory scrutiny and an accounting scandal, plummeted 9% without any announcements or changes. Hedge fund manager and Valeant defender Bill Ackman admitted yesterday that he had some misgivings about his stake in the company—but again, that was yesterday, and Valeant shares didn’t noticeably react at the time.

Eli Lilly (LLY), meanwhile, lost 6% during the day without making headlines. Allergan (AGN) fell 3% without so much as a peep, its shares declining well below the price Pfizer (PFE) has agreed to pay for them. And Teva Pharmaceutical Industries (TEVA) fell nearly 3% despite announcing the launch of a new injectable leukemia drug onto the market.

Unable to make sense of the carnage, Evercore ISI pharmaceutical analyst Mark Schoenebaum, famous in the industry for his weekly hand-holding conference calls with clients, held a webinar to address the confusion and offer experts’ thoughts on the selloff. As ISI trader Sean McDermott acknowledged when Schoenebaum put him on the line, “It’s probably the most questions we’ve ever been asked with regards to what’s going on in the space.”

But none of the analysts had a concrete theory on the market drop, though they discussed some of the speculation they were hearing from investors and clients. Perhaps people were bailing out of biotech stocks in advance of tonight’s Republican presidential debate, fearing that candidates might present plans to crack down on drug prices. Or they were worrying that China’s demand for medications might be slowing the way it had for iPhones. Blame Bernie Sanders, one listener suggested. But Schoenebaum wasn’t buying any of the excuses. “Frankly I reject all of that,” he said on the call. “I think what we’re seeing today is a continuation of something that probably started a long time ago. So today I don’t think there’s anything going on.”

By something that started a long time ago, he meant the ongoing uncertainty over pharmaceutical pricing regulation, general market jitters, lackluster earnings reports among biotech companies, and a dry spell of M&A deals and exciting new drugs. Still, it wasn’t possible to chalk up the crash in pharma stocks to overall macro conditions, because again, the sector that had been trading in tandem with the S&P 500 and Nasdaq suddenly took a left turn for the worst.

Naturally, investors want to know whether or not they should buy pharma stocks on this dip. But it’s hard to call the bottom of a market—especially when nobody even knows why it’s falling. While the sector’s valuations look attractively cheap, it’s possible, Schoenebaum said, that pharma stocks could drop another 15%.

In the meantime, he said, look for other signs that the industry is about to turn the corner—such as an activist investor swooping in to demand changes at a Big Pharma company, or a major drug company merger that could restart the pharma M&A wave. And there’s one thing that would surely mean biotech stocks had hit rock bottom: Their IPO market dried up completely. Investors are closely watching and waiting to see if Editas Medicine, a hot biotech company planning an IPO soon, will actually go public. If it cancels or postpones plans for a market debut, Schoenebaum thinks it’s probably safe to start buying biotech stocks again. His top pick? Pfizer, which held up relatively well amid Thursday’s industry selloff, falling only 1%.